1. Field of the Invention
The present invention pertains to a megasonic cleaning tank, and more particularly, pertains to a tank using two or more megasonic frequencies for cleaning of items, such as wafer chips. A unique transducer assembly generates the energy.
2. Description of the Prior Art
Ultrasonic frequencies have been used for a long time to cause cavitation in cleaning solutions. These frequencies have been in the thousand kilohertz or low hundred thousand kilohertz ranges. In recent years, higher frequencies have been used to affect the cleaning efficiencies of smaller particles in the submicron range. The frequencies called "megasonic" are generally understood to be in the range of one megahertz. However, use of a single megasonic frequency has not proved to be a "cure" for cleaning objects where different sizes of contaminates or particulates were abundant.
Also, in the past, the piezoelectric devices have been silvered, and epoxied onto a metal or quartz envelope. Epoxy bonding of a piezoelectric device to a sapphire or quartz mount has not, however, proven to be a good method for the following reason. Bubbles in the epoxy which were present in set-up or during use impede the sonic wave propagation. This in turn causes areas of non-cleaning to occur. The epoxy is a poor heat conductor which in use allows the piezoelectric ceramic device to heat up, which reduces the power output of the system.
Until now, quartz lenses have been used to shape the waves emitting from the transducers into the process tank. These are expensive to grind, and are generally fragile. The prior art transducers have been generally flat, and mounted on a flat "window".
The present invention overcomes the deficiencies of the prior art by providing a method of running a piezoelectric device at two distinct frequencies to further improve the cleaning efficiency wherein particles of a smaller diameter are affected by higher or different frequencies. Also provided is a method of low temperature bonding of the piezoelectric transducers to an inert envelope without the use of epoxy which allows use in ultra-high purity cleaning applications with higher cleaning reliability.